This invention relates to print cartridges for electrostatic printers and more particularly to a print cartridge with improved finger electrodes.
Electrostatic printers receive images in the form of electronically coded information and convert it to an output on a medium such as paper. Typically an electrostatic printer uses a print cartridge with a plurality of discharge sites which can be controlled to place electrostatically charged particles on a charge receiving surface such as a revolving print drum or moving belt to form charged dots which in turn make up a latent image. Typically, the receiving surface is comprised of an electrically conductive substrate coated with a dielectric coating to enable it to hold charged particles generated by the print cartridge.
In the following description, the charge receiving surface will be described for convenience primarily with reference to a drum.
Once the latent image has been formed on the drum, toner is applied to the image and subsequently transferred to the paper and fused at a nip between the receiving surface and a fusing roller. Advantageously, the print drum and fusing roller revolve on axes which subtend an angle of approximately forty-five minutes to aid in fusing the toned image to the paper.
Excess toner is removed from the drum by a scraper and any remaining latent image is then discharged by an erase head before the drum completes a revolution and starts the printing cycle again.
The electrostatic printer offers many advantages including relatively high-speed printing of computer generated images and the flexibility to print additional copies or to select either portrait or landscape images.
One type of print cartridge which has formed the basis for many modern electrostatic printers is described in U.S. Pat. No. 4,155,093 to Fotland et al which issued May 15, 1979. This type of cartridge provides for the generation of charged particles by an electrical gas breakdown in a field between two electrodes separated by a dielectric substrate. Rows of parallel and equally spaced driver electrodes are attached to one side of the substrate and run from one end of the cartridge to the other. Parallel and equally spaced finger electrodes are located on the opposite side of the substrate and extend diagonally across the driver electrodes. The finger electrodes define discharge sites in the form of a matrix of apertures corresponding to the points where driver and finger electrodes cross. An AC voltage may be applied to the electrodes to cause gas breakdown and charged particle production at edge structures associated with the apertures.
An improvement on this cartridge structure is described in U.S. Pat. No. 4,160,257 to Carrish which issued July 3, 1979. This patent teaches the use of a third or screen electrode separated from the finger electrodes by a dielectric layer. The screen electrode and dielectric layer both have a matrix of openings in alignment with the apertures in the finger electrodes. A DC field can be applied to this electrode to provide a lensing action for focusing charged particles generated by the cartridge to produce more precise charged dots on the printing drum.
In the cartridge structure described above, the apertures in the matrix are arranged in a series of diagonal rows coinciding with the finger electrodes. The apertures in each diagonal row are arranged to produce charged dots on a particular segment of the drum. Charged particle production can be initiated at each aperture as needed to place a charge on a corresponding point on the drum as the drum rotates past the aperture. The arrangement of apertures is such that it is possible to place charged dots anywhere within a selected zone on the circumference of the drum as it rotates past the cartridge to build up any selected image.
Electrostatic print cartridges of the type just described are well suited for producing both text and graphics although they do have limitations when producing large filled areas. In particular this type of cartridge tends to produce what has become known as a "Venetian blind effect". This effect has the appearance of striations extending in parallel in the direction of motion of the receptor surface past the cartridge and is pronounced when the surface is a drum. The striations take the form of strips of different intensity and result from variations in the charge deposited by the cartridge on the receptor surface. This can be caused by variations in performance of the cartridge across the active surface of the cartridge but is more noticeable as a result of the use of a flat cartridge with a drum of significant curvature. Such a combination inevitably results in the discharge apertures being further from the drum at the edge of the cartridge than they are at the centre of the cartridge. This results in variations in charge deposited and subsequent variation in toned image.
The Venetian blind effect is also caused by the fact that adjacent apertures deposit charge on the drum sequentially so that the most remote aperture discharges beside the next most remote and so on.